In certain fields, such as medical analysis (blood counts or flow cytometry), for example, fluid examining devices are used which are based on the interaction between the different components of a fluid and a light.
These devices generally comprise a measuring space having a compulsory passage for the fluid to be examined, a source supplying a selected light to optical illuminating means charged with illuminating the compulsory passage with this light, optical means serving to collect at least some of the light that has passed through the compulsory passage and deliver it to means serving to analyse the collected light in order to deliver the signals representative of data which it carries. A device of this kind is described in particular in French patent FR 2653885.
The data carried by the collected light relates for example to the forms and/or coefficients of reflection and/or coefficients of diffraction and/or coefficients of absorption of the various components of the fluid being examined. The data are obtained for example by transmission measurements well known in the art which need not be described here.
In the case of haematological analysis, for example, the examining device has to be able to distinguish blood cells automatically according to their category (chiefly leukocytes (or white corpuscles) and erythrocytes (or red corpuscles)), and the variety within their category (for example, lymphocytes, monocytes, neutrophils and eosinophils among the leukocytes, or reticulocytes among the erythrocytes).
In order that the examining device can distinguish between the different components suspended in the fluid to be examined, the components have to receive substantially the same intensity of light over the same period of time when passing through the compulsory passage in the measuring space.
In order to obtain a substantially uniform distribution of light over the entire surface defined by the compulsory passage it has been proposed to use an incoherent source such as an incandescent source coupled with a diaphragm and a so called Köhler optical assembly. This assembly consists in using a first lens to form the image of the source in the plane of the pupil of a second lens tasked with forming the image of a diaphragm in the plane of the compulsory passage of the measuring space.
A first drawback of this Köhler assembly device resides in the fact that it has a poor photometric yield, i.e. very little light is concentrated on the measuring zone, thus making certain analyses difficult, especially the analysis of biological cells.
In order to increase the yield it is possible to use high power sources, typically of several dozen watts. However, such sources generate considerable amounts of heat which are liable to interfere with the analyses and incur considerable costs.
A second drawback of this Köhler assembly device resides in the fact that it requires a set of lenses which cannot be easily be placed in the vicinity of the measuring space. In fact, as is known in the art, the smaller the size of the fluid expulsion nozzle, the larger it is possible to make the numerical aperture of the light be and the better the photometric balance. Consequently, for a fluid expulsion nozzle of a given diameter, the closer the illuminating optics are to the compulsory passage, the better the photometric balance.